Possible pH issue.

I don't know what you mean when you say you "dropped some lemon" into the plant... unless you spilled a lot of lemon juice on these leaves and didn't clean it up, this is not the cause, nor is it your pH (although that COULD be a little lower). This pattern is telling you this particular plant needs either more magnesium or more phosphorus. You can use this link to determine which nutrient is deficient: https://www.plantprod.com/news/9-symptoms-of-cannabis-plant-nutrient-deficiencies/ If you want to use only organic solutions to fix the issue, you can use Epsom salts for the magnesium and Langbeinite for the phosphorus... Since it is only on one plant (they're all individuals and there's nothing strange about the fact that only one of the plants is showing a deficiency), I would recommend using these as a top-dress ... so sprinkle a tablespoon of either Epsom salts or Langbeinite on top of the soil of this one plant, scratch it in as best you can, and that one plant will have what it needs and your normal watering/feeding routine won't have to change. And btw, only top-dress these once a month... Langbeinite is particularly slow to dissolve so this is the ideal means of using it. Good luck...

Coinsido con zookeeper. Agrega mas potasio y fosforo a tu dieta.suerte

Plants are looking for more food to grow you big fat buds. They are taking it from the leaves because you're not providing it to them. They need more phosphorus and potassium.